1. Field of Art
This description generally relates to energy storage, and particularly to energy storage using flywheels.
2. Description of the Related Art
Many energy sources, particularly clean energy sources such as wind turbines and solar panels, generate energy that does not temporally match the load experienced. In much of the developed world, energy generation follows experienced load, such that energy is provided as needed. Under circumstances of high load, techniques such as the use of peaker generators and spinning and non-spinning reserves on thermal generators allow for generation that matches high and variable load. However, despite the availability of such techniques, there are often instances where energy storage is important for meeting energy load.
Currently existing energy storage systems all have drawbacks of one form or another. Size, price, storage efficiency, efficacy, and safety are all concerns when designing an energy storage system. Generally, smaller size, lower price, reduced loss in both inputting energy for storage and extracting it for distribution, reduced losses for continuous operation, and safe disposal are all preferred characteristics of energy storage systems.
A flywheel is one type of energy storage system that stores energy as rotational kinetic energy. A flywheel rotor is a weighted, rotationally symmetric mass that spins while physically coupled, directly or indirectly, to a motor/alternator that itself is electrically coupled to a converter, such as a back-to-back inverter system, constituting an AC-AC conversion subsystem. When power is received for storage, the rotor is driven, increasing the rotational speed of the flywheel rotor. When power is to be extracted, the flywheel rotor drives the motor/alternator. The faster a flywheel rotor can spin, the more energy it can store, but the more stress is induced on the rotor. Generally, the amount of stress a rotor is able to sustain while operating is a function of the design, materials, and processes used to make the rotor. Specifically, the amount of stress that can be sustained depends on a combination of the rotor material's yield strength, fracture toughness, maximal intrinsic defect size, cyclic fatigue characteristics, and the rotor's shape, among other factors. Generally, a flywheel's bearing and suspension subsystem is designed to minimize energy losses due to friction, and other loss sources.
Cost relative to the amount of energy that can be stored is of particular importance for a flywheel system. The cost of a flywheel system can be roughly divided into two portions, the cost of manufacturing the flywheel rotor, and the balance of system costs for supporting elements such as bearings, mountings, enclosure, etc. In the past, flywheel rotors have been very expensive to manufacture. As a result, flywheel systems have primarily been used in applications involving only seconds to minutes of energy storage, as it was simply too costly to either manufacture a single rotor that can store tens to hundreds of kWh of energy, or to use many individual rotors that are cost inefficient with respect to the balance of systems costs for the supporting elements used in conjunction with the rotors.
Some existing flywheel rotors are made of common, low alloy steels such as American Iron and Steel Institute (AISI) 4340 and AISI 4140. These steels have low costs and other desirable properties, however such rotors are limited to thin sections due to limitations in through-hardenability, which is required to achieve a useful yield strength and therefore that can handle a significant amount of stress. For example, although these rotor materials can achieve ultimate tensile strengths (UTSs) of 2 gigapascal (GPa), and fracture toughness of 40 megapascal square root meter (MPa·m0.5), such rotors are limited to maximum cross-sectional thicknesses of 3-6 inches.
Other steel flywheel rotors are made with high-alloy steels such as maraging steels, Aermet steels, and some stainless steels. These flywheel rotors are able to sustain higher stresses throughout cross-sectional thicknesses greater than 6 inches. These rotors achieve these stresses without the need for multiple separate sections, but are cost prohibitive due to the high content of expensive alloying elements such as nickel and cobalt. Other modern flywheel rotors are made of carbon fiber and therefore allow for significantly higher working stresses, however the high cost of carbon fiber and the ancillary components needed to achieve the corresponding higher rotational speeds makes carbon fiber rotors prohibitively expensive, despite their high working strength-to-weight ratios.